Radiometric film



Dec. 5, 1967 R. WITHNELL RADIOMETRIC FILM Filed Sept. 2, 1964 TRANSMISSION- PERCENT WAVE LENGTH MICRONS F|G.l INFRA RED TRANSMISSION OF COATED PLASTIC FILMS United States Patent Ofiice ABSTRACT F THE DESCLQSURE A thin 411' source carrier film, and method for making the same for a radioactive source for radiometric measurements, having an electrically conducting coating with high infra-red light transmission.

This invention relates to radiometrics, and in particular, to novel method and apparatus for providing a thin source carrier film having a conductive coating for supporting a radioactive source for radiometric measurements in a 41r counter.

In radiometric studies a need exists for a durable source carrier film for a radioactive source for radiometric measurernents in 411' counters. These films have a density of only from two micrograms/cm. to about 100 micrograms/cm. with conductive coatings thereon to prevent small, static electrical charges from building up on the films. These coatings have a density of from about 6 to 10 micrograms/cm. and are required on both sides of the film when the film has a density of about micrograms/cm. or more. The radioactive source for counting comprises a few drops of an aqueous, acid or basic solution or suspension that is placed on the center of the coated film and quickly dried with an infra-red light in order to prevent these drops of solution or suspension from spreading out too much on the film. This drying, however, often results in breaking the film with a consequent expensive and time consuming loss of the film and the source, which has often been difficult or impossible to replace. It is universally recognized, therefore, that an improved system and film are required to avoid this breakage.

It has been discovered in accordance with this invention that this breakage hasbeen due to a low infra-red transmission by the coated films during the above described drying and that the h retofore known problems can be overcome by providing a suitable conducting coating having high infra-red transmission.

It is an object of this invention, therefore, to provide an economical and practical 4ncounting source carrier film having a conducting coating thereon with high near infra-red light transmission;

It is another object to provide a uniform palladium coatingon a 41- counter film;

It is another object to provide an improved 41.- counter film;

It is another object to provide an economical, simple and practical method for coating a thin plastic source carrier filmwith a thin, electrical conducting coating having high near infra-red light transmission;

It is another object to provide a thin 411' counting source carrier film having a conducting coating on one side with a high transmission of infra-red light in the near infra-red band of wavelengths from about 2.5 to 7 microns;

It is another object to provide a thin 4'rr counting source carrier film having a conductive coating on both'sides with a high transmission of infra-red light in the near infra-red band of wavelengths fl'OIll' about 2.5 to 7 microns.

The foregoing objects are achieved by heating pal- Patented Dec. 5, 1967 laidium in a high vacuum to coat the 41r source carrier film by molecular emission and condensation with a transparent conducting network. With the use of small amounts of palladium at the proper temperature, distance and vacuum between the heated palladium and the 41r source carrier films, as described in more detail hereinafter, the desired high infra-red transmission and durable conducting source carrier films are produced.

The above and further novel features of this invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

In the drawings where like parts are marked alike:

FIG. 1 is a graphic comparison of the low infra-red transmission of 41r source carrier film that breaks when exposed to a 250 watt infra-red light ten inches away and the high transmission of the coated 41r source carrier film of this invention, which does not break when exposed to the same or more infra-red radiation from ten inches away down to four inches away;

FIG. 2 is a partial schematic drawing of the apparatus employed in the method of this invention;

FIG. 3 is a partial cross-section of the support for the tungsten wire and palladium wire of the apparatus of FIG. 2;

FIG. 4 is a partial schematic view of the infra-red heating lamp apparatus for drying the source solution or suspension applied to the film coated with the apparatus of FIG. 2.

The disintegration rate determination of radioactive samples by 4n gas-ion counting involves the use of counters connected to radiation detectors above and below a thin source carrier film for counting the number of fissions from the source on the film. One 411' counter is described and shown in FIG. 1 on pp. 279-280 of Nuclear Instruments and Methods, 14 (1961). The films for these counters have minimal superficial density and low aver age atomic number in order to reduce absorption and scattering of radiation; adequate mechanical strength to Withstand shocks received during normal handling such as receiving the radioactive solutions or suspensions; adequate chemical resistance to reagents present during the rapid evaporation of these solutions or suspensions; and a large area for receiving the radiation samples. The samples may be any radiation source, such as metallic samples irradiated in the Brookhaven 33 bev. Alternating Gradient Synchrotron or Cosmotron.

Synthetic resins and plastics have generally been used for the source carrier films. Suitable materials, comprise, polyvinylformal, such as the Formvar brand of this material, cellulose nitrate, cellulose acetate, polyethylene terthalate, or polyvinyl chloride-acetate copolymer. These materials are used alone or are laminated with polystyrene. One suitable brand of polyvinyl chloride acetate copolymer is VYNS resin made by the Bakelite Co. of New York, NY. This material is commercially available in superficial densities of 1-100 gm/ch1 10 gm/cm. corresponding to a thickness of about 7 m These films also have areas up to cm. and both singly and in laminations have good tensile strength and excellent resistance to acids; alkalies and many organic reagents. These films are stable, however, up to only between about 100-150" C. depending on their thickness.

tion 'or transmission. For reflection determinations, as is well'lcnown, the films characteristically reflect light, owing to the occurrence of interference and reinforcement between the light reflected from the top and bottom surfaces of the film. The conditions for reinforcement and destructive interference for light of wavelength A are given by:

Reinforcement: (n+ /2) \=2;td cos Destruction: n \=2;td cos 0 where [L is the film refractive index (for VYNS=1.5), d is the film thickness, 0 is the angle of reflection and n is an integer. For photic transmission measurements, a Beckman Model DU Spectrophotometer, balanced against air is used at several wavelengths according to the thickness range and accuracy required. This spectrophotometer is properly calibrated against material of known thickness accurately to determine the film thickness and density. This spectrophotometer also determines the infra-red transmission of the films.

Referring to FIG. 1, the film coated in accordance with this invention has a 90% infra-red transmission for a conductive coating on one side, as shown by line (a) in this figure, the infra-red transmission being measured in the near infra-red wavelength range of from 2.5 to 7 microns. An 80% infra-red transmission is also obtained for a coating on both sides in accordance with this invention. In normal handling, neither of these films breaks during assembly, coating, drying, or use whereas films having a 75% or less infra-red transmission are subject to breakage during these above mentioned operations. Line (b) in this figure, for example, shows an actual film having a 75% infra-red transmission. This latter film broke during infra-red drying in which the liquid radioactive sample Was dried to prevent the sample from spreading out too much on the coated film. These films when uncoated had an infra-red transmission of about 98.5% and a 250 watt lamp 10 inches away was used for all these determinations. The lines in the graph in FIG. 1 of the examples used show small infra-red transmission valleys or characteristic low points. These are the results of interference reflection and not absorption of infra-red by these examples of the films. This interference reflection is due to the fact that small amounts of the infra-red light are reflected in rays from the top and bottom surfaces of the coated films and certain of these reflected rays from the top and bottom surfaces of the coated films cancel due to destructive interference. These characteristic infra-red wavelength transmission low points change up or down from 2.5 to 7 microns with greater or lesser thickness of the film and are an exact measure of the film thickness.

Palladium is evaporated quite easily to obtain a suitable high infra-red transmission of at least 80% by the apparatus shown in FIG. 2. Glass bell jar 11 is a minimum of twenty-four inches high and has as its bottom a one inch or larger pipe 13 connected through a valve 15 and liquid nitrogen trap 17 to a mercury diffusion pump 19 having a suitable heater 21. A rotary pump 23 having at least a c.f.m. capacity, connects on one side to bell jar 11 through a roughing valve 25 and on the other side to a laboratory hood (not shown). The diffusion pump 19 also connects with the rotary pump 31 through pipe 27 and valve 29. A thermocouple gage 31, an ionization gage 33, and an air inlet valve 35 are also provided for control purposes. The film 41 for coating is supported by a circular aluminum frame 42 that hangs from adjustable supports 43 and 45, which can adjust the height of film 41 above the heater 49.

Referring to FIG. 3, heater 49 has a 110 volt alternating current source 51 connected through a variac 53 and a transformer 55 rated for 100 amp continuous use. The output terminals 57 and 59 of this transformer 55 connect to two threaded copper rods 61 and 63 supported in bell jar 11 by support 65 in vacuum tight arrangement therewith. These rods 61 and 63 are insulated from their support by polytetrafluoroethylene bushings 71 and 73 having neoprene O ring seals 75and 77 interposed between these bushings and the support 65. Theaded insulators 79 and 81 tighten on rods 61 and 63 against the top of the bushings. Other insulators 82 and 83 fit over the bottom of rods 61 and 63 against support 65 while conducting fittings 84 and 85 hold the respective output leads 87 and 89 from transformer 55 against rods 61 and 63.

A tungsten filament 49 stretches between rods 61 and 63 and the cold coating material is placed on this filament 49. The tungsten filament 49 advantageously has a form or shape to direct the vaporized coating material against the film 41 and to this end, in one example, the filament 49 is a flat ribbon which is parallel to the film 41.

It has been found that because tungsten has a tendency to combine with other materials at high temperatures, that all the coating material has not left the filament 49 by molecular emission or impurities were present in the coating on film 41 which could influence the 411- counter measurements. These problems were solved by using palladium at 1570 C. since this temperature vaporizes the palladium without combining it with the tungsten filament 49.

Because this temperature is high enough to break the film, the film 41 must be raised above the filament 49. The film must also be high enough not to condense substantially in crystalline form as this is inefficient and cuts down the transmission of the infra-red drying rays. On the other hand the film must not be raised too high or the molecular emission will not be sufficient properly or efficiently to coat film 41. These problems are solved, in one example, by energizing the tungsten filament 49 in a high vacuum to heat a 10 milligram foil of pure palladium quickly to 1570 C., the palladium being in conforming contact with the filament 49 so that the palladium travels by molecular emission in an uninterrupted, straight path across a uniform 50 cm. gap between the filament 49 and a circular film 41 having a diameter of 1% inches. The foil 50 in this example is a ribbon foil .005 inch thick and wide. The pure palladium in this example has high molecular emission energy, condenses uniformly on the film 41 with a density of 6-10 micrograms/cm. and mechanically, strongly attaches the palladium to the film substantially without convection currents. This operation takes about two to three minutes and the filament 49, since it does not combine with the palladium, is then completely clean for the beginning of another coating operation.

The palladium condenses first on film 41 in small uniformly spaced islands, and as the condensing continues these islands grow small uniform arms which connect in a network across palladium free areas on film 41 to form uniform conducting connections all the way across the film 41. At the low temperatures of the film 41 in accordance with this invention, the palladium coating on film 41 tends to be substantially amorphous rather than crystalline. Also, the islands tend to be smaller at these low temperatures than at the high temperatures caused, for example, by putting the film 41 and filament 49 closer together. Thus this system is efiicient since the amorphous state requires less coating material to get a sufiiciently conducting coating. Also, the large palladium free areas between the islands can freely transmit the infra-red drying rays.

The vacuum jar 11 is released slowly through valve 35 whereupon the jar 11 is raised and the film 41 is removed or turned for coating the other side if desired. For the latter, ten milligrams of palladium foil are placed on filament 49 as described above, the jar 11 is reseated and reevacuated and the palladium is properly heated as described. The proper vacuum is provided from below by sequentially operating valves 35, 29, 25 and 15. First valves 15, 29 and 35 are closed and valve 25 is open. The pump 23 evacuates the jar 11 to a pressure of about 15 microns of mercury whereupon valve 25 is closed and valves 15 and 29 are opened. Then diffusion pump 19 evacuates the jar 11 to a pressure of about 10 Torr.

Filament 49 then heats the palladium by adjusting the variac 53 to supply current to filament 49 to heat the palladium to 1570 C. for about two to three minutes. Valve 35 is then opened to release the vacuum in jar 11, for the beginning of another cycle "for the coating of further like films 41 as described above.

In another system for obtaining the described palladium vaporization and uniform coating by molecular emission substantially without convection currents, a 1 mm. X 2 mm. piece of palladium is placed on a palladium surface 50 cm. from the film 41, the crucible is placed on a high heat transfer copper plate, and the palladium is vaporized at 1570 C. by an electron beam in jar 11 at Torr. One suitable electron gun is the model 980-0001 electron beam gun made by the Varian Co. of Palo Alto, Calif. In these devices, electrons are boiled off a filament at up to 4000 volts and the electrons are focussed on the palladium piece by suitable magnets.

In applying the radioactive sample to the coated film 41, a few drops of an aqueous, acid or other solution or suspension containing the radioactive sample is applied to film 41 with an eye dropper while the film is supported by aluminum disc 42, which is supported on a wire support 111 as shown in FIG. 4. This film is then quickly heated by a commercial 250 watt infra-red lamp 112 supported on movable mounts 113 about ten inches above the film 41 until the solvent or suspension liquid is dried by evaporation. One suitable lamp is the 250 watt model lamp made by the General Electric Company. The film is then ready for radiometry determinations in a conventional 41.- counter, such as the one mentioned.

It has been found that this described coated film 41 provides a low molecular weight for low radiation absorption and scattering. This film 41 of this invention also has sufficient charge conductivity to its aluminum ring support to eliminate charge build-up on the film. The palladium coated films of this invention also have sufliciently high infra-red transmission and low superficial densities to make them advantageous for receiving samples for 41r radiometry determinations and for infra-red drying thereon of conventional sample solutions and suspensions. Moreover, the palladium is sufficiently unreactive with tungsten and has a low enough melting point so that it can easily, simply, economically and practically be vaporized with a tungsten filament or an electron beam and condensed in pure form in an open coating network to form a thin 6-10 microgram/cm. on existing low temperature 41:- plastic o resin film substrates. Additionally, actual tests have shown an approximate 100% decrease in breakage under normal handling procedures with the system and coated film of this invention over the heretofore known systems and coated films.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that numerous modifications or alterations may be made therein without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. The method of coating a thin 41r source carrier film with a thin conducting coating in a vacuum jar, the film comprising a thin, polyvinyl chloride-acetate copolymer film having a density of up to 15 micrograms/cm. comprising placing a 10 milligram fiat ribbon of solid palladium in conforming contact on a cold foil-shaped tungsten filament in said jar, placing said 41r source carrier film with one side uniformly cm. above said palladium, evacuating said jar around said source carrier film to about 10 Torr, and supplying alternating electrical current to said tungsten filament to heat said palladium to 1570" C. to avaporize said palladium by molecular emission to deposit said palladium by condensation in a uniform conducting amorphous network on said one side of said source carrier film for providing a thin conducting film with high near infra-red light transmission and low radiation absorption and scattering.

2. The 41r source carrier film produced by the method of claim 1.

3. A thin 41r source carrier film having an electrically conducting coating thereon with a high infra-red transmission of wavelengths from about 2.5 microns to about 7 microns, in which said film is a 1% inch diameter polyvinyl chloride-acetate copolymer film having a density of up to 15 micrograms/cm. and has a thin palladium coating having a density of 6 to 10 micrograms/cm. on one side of said film with 90% infra-red light transmission.

4. A thin 41r source carrier film having an electrically conducting coating thereon with a high infra-red light transmission of wavelengths from about 2.5 microns to about 7 microns, in which said film is a 1% inch diameter polyvinyl chloride-acetate copolymer film having a density of 15-100 micrograms/cm. and has a thin palladium coating having a density of 6-10 micrograms/cm. on both sides of said film with infra-red light transmission.

No references cited.

WILLIAM L. JARVIS, Primary Examiner. 

1. THE METHOD OF COATING A THIN 4$ SOURCE CARRIER FILM WITH A THIN CONDUCTING COATING IN A VACUUM JAR, THE FILM COMPRISING A THIN, POLYVINYL CHLORIDE-ACETATE COPOLYMER FILM HAVING A DENSITY OF UP TO 15 MICROGRAMS/CM.2, COMPRISING PLACING A 10 MILLIGRAM FLAT RIBBON OF SOLID PALLADIUM IN CONFORMING CONTACT ON A COLD FOIL-SHAPED TUNGSTEN FILAMENT IN SAID JAR, PLACING SAID 4$ SOURCE CARRIER FILM WITH ONE SIDE UNIFORMLY 50 CM. ABOVE SAID PALLADIUM, EVACUATING SAID JAR AROUND SAID SOURCE CARRIER FILM TO ABOUT 10**6 TORR, AND SUPPLYING ALTERNATING ELECTRICAL CURRENT TO SAID TUNGSTEN FILAMENT TO HEAT SAID PALLADIUM TO 1570* C. TO AVAPORIZE SAID PALLADIUM BY MOLECULAR EMISSION TO DEPOSIT SAID PALLADIUM BY CONDENSATION IN A UNIFORM CONDUCTING AMORPHOUS NETWORK ON SAID ONE SIDE OF SAID SOURCE CARRIER FILM FOR PROVIDING A THIN CONDUCTING FILM WITH HIGH NEAR INFRA-RED LIGHT TRANSMISSION AND LOW RADIATION ABSORPTION AND SCATTERING. 